Evaporative Light Scattering (ELS) detection is a method of detecting samples that have been previously separated by chromatography methods such as High Performance Liquid Chromatography (HPLC), Gel Permeation Chromatography (GPC) and Gradient Polymer Elution Chromatography (GPEC). Evaporative Light Scattering Detection is preferably used when the samples to be analysed are less volatile than the mobile phase (solvent). The technique is useful in the analysis of many substances including carbohydrates, lipids and fatty acids, underivatised amino acids, pharmaceutical compounds, surfactants, polymer blends and copolymers.
ELS detection is a three stage process:    1. Nebulization of the chromatography eluent, either as mobile phase (solvent) or eluent containing a less volatile sample, to form an aerosol or plume of uniform droplets. Most commercial ELS detectors produce this plume by introducing the eluent to a high velocity stream of nitrogen or other inert gas in a pneumatic nebulizer, but it can also be achieved by methods such as passing the eluent through a high frequency vibrating capillary or impacting it upon an oscillating plate or ‘horn’ (ultrasonic nebulizer).    2. Evaporation of the mobile phase to generate a plume of non-volatile solute (sample) particles. This occurs in a heated tube commonly referred to as a ‘drift tube’ or ‘evaporator’.    3. Optical detection of the light scattered by the solute particles from an incident light beam. In principle, the detector response is proportional to the mass of solute passing through the light beam.
Those skilled in the art will readily appreciate that any mobile phase remaining as un-evaporated droplets in the plume will produce an undesirable response from the detector in the form of ‘baseline noise’ as it passes through the light beam. In order to minimise this effect it is common practice to remove from the plume the largest droplets (which are the most difficult to evaporate). This is most commonly achieved by utilising an impact trap; the simplest form of which is that of a drift tube of smaller diameter than the natural form of the aerosol plume produced by the nebulizer. The large droplets are less mobile and more “ballistic” in nature than the small droplets so impact upon the walls of the drift tube. Once impacted upon the walls the liquid can either be channelled to waste or boiled off by heating the drift tube above the boiling point of the liquid. Alternative designs of impact trap in common use include drift tubes with bends or changes in cross section or balls, plates or flaps in the immediate path of the primary aerosol.
It can be seen therefore that to the three principal stages of Evaporative Light Scattering Detection (ESLD) a fourth has been added; the removal of the larger droplets and the selection of the remaining portion of the nebulized plume for detection. This stage is employed in every current commercially available ELSD.
Hereinafter the following definitions are used:    Primary aerosol The mixture of eluent droplets and nebulizing gas as formed by the nebulizer.    Secondary aerosol The portion of the mixture of eluent droplets and nebulizing gas remaining after the impact trap and physically selected for evaporation.    Tertiary aerosol The mixture of sample residue and gas exiting the evaporator after drying has taken place.
The response of the ELSD is dependent on the concentration of sample droplets or particles in the tertiary aerosol, relative to the volume of gas. In order to increase the sensitivity much effort has been made to reduce the quantity of gas required by the nebulizer to produce a stable aerosol without diluting the sample.
The vapour loading in the plume is a function of nebulizer temperature. Higher nebulizer temperatures permit greater sample loading of the plume resulting in more complete evaporation and greater sample density at the detector. The gas immediately after nebulization will be saturated, therefore complete drying of the sample droplets and vaporization of the mobile phase in the secondary aerosol will be impossible unless the temperature of the gas is subsequently raised. This temperature rise will increase the saturation vapour pressure of the eluent in the gas sufficiently to absorb all the liquid in the droplets. For this reason, the drift tube in an ELSD is normally heated.
Depending on the design of the instrument, the efficiency of the nebulizer and the volatility of the eluent, it is frequently necessary to operate with evaporator temperatures in excess of the boiling point of the eluent. As an example, the PL-ELS 1000 (manufactured by Polymer Laboratories Ltd, Church Stretton, Shropshire, UK) features a nebulizer of a highly efficient type, requiring nebulizer gas flow rates of the order of only 1 standard Liter per minute (SLM) to produce a stable primary aerosol with eluent flow rates of up to 2 ml/min. In this circumstance (using the example of a water-based eluent) it is generally necessary (in order to maximize the signal to noise ratio) to operate with the temperature of the evaporator set to 120° C.
However, it is frequently not desirable to raise the temperature as this will cause evaporation or degradation of volatile samples and hence loss of sensitivity of the detector for these samples.